Transparent disks

The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure in vacuo to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophili-... 

While a MSssbauer effect cell with an operating range from 77 to 800 K is routinely used in catalytic studies, temperatures lower than 77 K may not be as commonly encountered. In this case, it may be advantageous to pretreat the sample and then seal it between y-ray transparent disks (e.g., Mylar) in... 

Figure 5.39 depicts a rotated electrode system that reverses the roles of the light and the electrode from that of preceding assembly. Here an intense beam of light shines through the transparent disk to produce a photochemical reaction. The products of the photochemical reaction are swept past the disk electrode, where they may be monitored for their electroactivity. 40... 

The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophilization. The mixture is pressed with special dies under a pressure of 10,000-15,000 psi into a transparent disk. The quality of the spectrum depends on the intimacy of mixing and the reduction of the suspended particles to 2 gm or less. Microdisks, 0.5-1.5 mm in diameter, can be used with a beam condenser. The microdisk technique permits examination of samples as small as 1 fxg. Bands near 3448 and 1639 cm-1, resulting from moisture, frequently appear in spectra obtained by the pressed-disk technique. 

KBr disks here the finely ground solid compound is mixed with anhydrous KBr and squeezed under pressure. The KBr becomes fluid and forms a disk containing the solid compound dispersed evenly within it and suitable for obtaining a spectrum. The advantage of the KBr disk technique is the absence of the spectrum from the mulling liquid, but the disadvantages are the equipment required (Fig. 28.4) and the practice required to obtain suitable transparent disks, which are very delicate and rapidly absorb atmospheric moisture. 

Solids are usually ground with a material such as potassium bromide and compressed into a pellet. Moisture must be absent, and the transparent disk is placed in the window of the spectrometer. In general, however, measurement of intensity of absorption in the solid phase is unreliable due to scattering and reflection losses, and a uniform distribution of sample in the pellet is difficult to achieve. One method of handling solutions is to allow them to soak into a KBr wedge, evaporate the solvent, and compress the tip into a microdisk or pellet (C26) alternatively the sample (1 /tl) may be either placed directly on the KBr disk (B13) or mixed with a little KBr powder and subsequently incorporated in the disk. These microdisk techniques can be used for the examination of gas chromatograph effluent. For multiple analyses, an enclosed turntable system loaded with the disks can be used. 

Fig. 5 Cascading flow occurs in large tumblers or during tumbling of fine, but freely flowing, grains. This snapshot shows a 1 m diameter transparent disk tumbler partially filled with colored 500 im irregular grains. The free surface is manifestly not flat, and the cascading layer is thin and nearly uniform with distance along the flowing surface.

ABSTRACT A rapid and precise X-ray fluorescence method has been developed for the multielement analysis of gypsum and gypsum products. Gypsum specimens are calcined at IOOO°C and then fused with sodium tetraborate flux into flat and transparent disks. The choice of a suitable flux system for the specimen preparation is critical because of a rapid decomposition of anhydrite. CaSO,. in lithium ba fluxes at temperatures above 95O C. This decomposition causes not only visible imperfections in the didi surface but also alters considerably the concentrations of the major elements, calcium and sulfur. The procedure used for a fast setup of ten element analysis of gypsum on the Philips PW-1400 spectrometer utilizing synthetic standards and off-line calculated alpha coefficients is presented. Calibrations carried out with chemically analyzed specimens and their mixtures are compared lo those performed with synthetic standards prepared by blending pure chemicals and anhydrite into the flux. 

Sodium tetraborate (Spectroflux 200 , Johnson and Matthey Co.) was tried with much greater success. Disks produced by fusion of 0.5000 g of calcined gypsum and 6.00 g of sodium tetraborate (Table 1) produced absolutely clear and transparent disks with perfect surfaces. The use of a higher specimen to flux ratio was dictated by the lower solubility of anhydrite in molten borax. 

Measurements in the infrared therefore must be made with the substance present in a material that does not absorb. Certain organic solvents are used frequently for this purpose. Alternatively, the solvent is eliminated completely. A common modern technique is to disperse the sample in a suitable inorganic salt, usually potassium bromide. The sample is mixed with the powdered crystalline salt, which is then pressed into a transparent disk measuring 0.5 mm in thickness and 10 mm in diameter. The disk is then mounted in a holder which is supported in the beam of the infrared instrument. There are some experimental difficulties which can be overcome by a skillful investigator. Since aqueous systems cannot be used in such experiments, infrared spectroscopy has no direct value in the study of biological systems, which are always aqueous. The usefulness of spectroscopy to the biologist is in the study of substances that have been extracted from biological systems. 

For examination of infiured spectra of the copolymers a sample (1 mg) is ground with KBr (0.2 gm) and compressed to a transparent disk at 16,000 blm. A similar blank is used in the reference beam. Although quantitative work is not feasible in examination of the spectra, particular attention is given to relative... 

Pellets are used for solid samples that are difficult to melt or dissolve in any suitable solvent, or which have to be measured in their native solid state. The sample is finely ground and mixed with dry potassium bromide (or other alkali halide) powder. The usual analyte/KBr ratio is ca. 1 100. Grinding and mixing can be done with an agate mortar and pestle or with a vibrating mill. The mixture is then pressed into a transparent disk in an evacuable die for 2 min at a pressure of... 

Next, 1 to 2 mm of the tip is cut off with a sharp scalpel, mashed on a clean metal surface, and pressed into a transparent disk with a microdie. Satisfactory spectra for qualitative analysis are obtained from 10 to 50 fig of sample when a 1.5-mm-diameter micro-KBr-die and a beam-condensing unit are used. 

The disk centrifuge consists of a transparent disk-shaped container that spins at a speed of 500-15 000rpm. The particles to be measured are introduced onto the spinning disk using a so-called buffered line start method in which a spin fluid is made up so that a density and viscosity gradient is created within it. This prevents streaming or settling of the particles in the spin fluid. 

One of the most popular systems is a laser disk for audio known as a compact disk (CD), in which PCM signals are recorded on the rear side of a plastic transparent disk and laser light reads them as shown in Fig. 20. A digital video... 

Kaye Disk Centrifuge. A device for particle-size analysis (1-50 pm) by means of a beam of light passed through a liquid suspension of the particles while the suspension is rotated in a transparent disk-shaped centrifuge. The prototype was designed by B. H. Kaye (Brit. Pat, 895,222,2/5/62) now made by Coulter Electronics Ltd., St. Albans, England. Keatite. A form of silica resulting from the crystallization of amorphous precipitated silica at 380-585°C and water pressures of 350-1250 bars sp. gr. 2.50. Named from its discoverer, P. P. Keat (Science, 120, 328,1954). 

More universal FTIR detection can be achieved when the mobile phase is removed from the sample prior to spectral analysis. The sample fiactions are then measured in a pure state without interference from solvents [6b, 6c, 8]. A widely used commercial LC-FTIR interface is manufactured by LabCormection. The LC-Transform strips volatile mobile phases by nebulizing the HPLC eluent and spraying it onto a rotating germanium (IR-transparent) disk forming a sohd, time-resolved deposit. In an off-line second step, the disk is placed in the sample compartment of a standard FTIR spectrometer and IR spectra of KBr qtiality can... 

The alkali halides listed are quite similar in many of their physical properties and differ primarily in the low-wavenumber cutoff. NaCl is the hardest and least hygroscopic and Csl the softest and most hygroscopic. Powdered KBr, KCl and Csl can each be compressed into transparent disks. When a small quantity of a very finely... 

A very popular technique for mnning solids is the KBr disk technique. Here a few milligrams of sample are very finely ground and then mixed with 50 to 100 parts of dry KBr powder. The mixture is placed in a special device and compressed into a disk at high pressure. If all goes well, a transparent disk results, which is put into the spectrometer and run. Commercial KBr disk makers are available in many forms. Some are activated with wrenches or levers, while others are used with a hydraulic press. Some can be evacuated, which gives the disk transparency a longer lifetime, but this is not necessary if the disk is used promptly. 

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